Also run tests against docker container #409

Currently

Almost all integration tests run against local server instances (server.listen() on the main process) of the proxy and the mock.

This means that problems specific to the Docker image build are not caught by the tests. Issues like #401 will go unnoticed.

This change will...

• Build the Docker container before the tests are run.
• Re-run integration tests also against the Docker container of openapi-cop.